1. Technical Field
The present application relates to an optical spectrum analyzer for measuring an optical spectrum of incident light by dispersing the incident light into a specific wavelength component and measuring the intensity of the dispersed light.
2. Related Art
Conventionally, an optical spectrum analyzer for measuring wavelength characteristics of an active device such as a laser apparatus has been known. The optical spectrum analyzer disperses inputted incident light with a diffraction grating set to an angle corresponding to a sampling wavelength inside a monochromator, and continuously measures the optical intensity. This enables an optical spectrum to be measured.
FIG. 3 illustrates a configuration diagram showing one example of a conventional optical spectrum analyzer. This optical spectrum analyzer includes an input unit 1, a monochromator 2, a diffraction grating 21, an optical receiver 6, an amplifier 7, an A/D converter 8, a diffraction-grating control unit 3, a control unit 400, a calculator unit 500, and a display unit 9. To the monochromator 2 is inputted incident light from the input unit 1. The diffraction grating 21 is included inside the monochromator 2. The optical receiver 6 receives dispersed light beams obtained by the dispersion by the diffraction grating 21. The amplifier 7 amplifies the output of the optical receiver 6. The A/D converter 8 A/D-converts output of the amplifier 7 to thereby measure the optical intensity of the dispersed light beams. The diffraction-grating control unit 3 drives (rotates) the diffraction grating 21. The control unit 400 controls timing of sampling of the optical intensity for the dispersed light beams. The calculator unit 500 generates optical spectrum, based on sampled optical intensity data. The display unit 9 displays the generated optical spectrum.
The incident light, which is input from the input unit 1 to the monochromator 2, is dispersed by the diffraction grating 21 inside the monochromator 2. The dispersed light beams enter the optical receiver 6.
The wavelength of the dispersed light beams depends on the relative angle of the diffraction grating 21 with respect to the incident light. The calculator unit 500 calculates a target diffraction-grating angle corresponding to the sampling wavelength for acquiring the optical intensity, based on set measurement conditions such as sampling start/end wavelengths, the number of measurement points and the like. The calculator unit 500 writes, on a memory inside the control unit 400, the target diffraction-grating angle data (Dsmp) set before the measurement start.
The diffraction-grating control unit 3 drives the diffraction grating 21 in a range from an angle corresponding to the sampling start wavelength to an angle corresponding to the sampling end wavelength, and transmits a current diffraction-grating angle (Dgrt) to the control unit 400.
The control unit 400 compares the current diffraction-grating angle Dgrt inputted from the diffraction-grating control unit 3 and the target diffraction-grating angle stored on the memory. When the diffraction-grating angle Dgrt reaches the target diffraction-grating angle, the control unit 400 generates a trigger signal Strg indicating the timing of the sampling. The A/D converter 8 receives this trigger signal Strg and acquires optical intensity data Dlum at the time of the reception to transmit it to the control unit 400. The A/D converter 8 repeats this processing every time it receives the trigger signal Strg. This allows the control unit 400 to acquire the optical intensity data Dlum of all the sampling wavelengths.
The calculator unit 500 receives the optical intensity data Dlum through the control unit 400, and acquires optical spectrum data indicating wavelength versus optical intensity. The calculator unit 500 further applies correction of a level value to the acquired optical spectrum data to thereby generate display data and display this display data on the display unit 9.
FIG. 4 illustrates a diagram showing a configuration of the control unit 400. The control unit 400 includes a memory 410, a comparator 420, and an incremental counter 430 that specifies an address of the memory 410.
First, before the measurement start, the calculator unit 500 writes the plurality of pieces of target diffraction-grating angle data Dsmp in accordance with the plurality of measurement conditions in the memory 410 in order from an initial address. During the measurement, the incremental counter 430 specifies the address of the memory 410 from the initial address. The memory 410 outputs the data at the specified address to the comparator 420.
The comparator 420 compares the target diffraction-grating angle data Dsmp inputted from the memory 410 and the angle information Dgrt indicating the current angle of the diffraction grating 21. When the diffraction-grating angle Dgrt reaches the target diffraction-grating angle, the comparator 420 generates the trigger signal Strg. The trigger signal Strg is transmitted to the A/D converter 8 and the incremental counter 430. Upon receiving the trigger signal Strg, the incremental counter 430 increments a count value. Thereby, the address of the memory 410 to be specified is updated, so that the target diffraction-grating angle Dsmp to be outputted to the comparator 420 is updated. On the other hand, upon receiving the trigger signal Strg, the A/D converter 8 acquires the optical intensity Dlum corresponding to the sampling wavelength.
In this manner, the calculator unit 500 calculates all of the plurality of pieces of target diffraction-grating angle data Dsmp corresponding to the plurality of sampling wavelengths in accordance with the plurality of measurement conditions before the measurement start. The calculator unit 500 sequentially writes these pieces of data from the initial address on the memory 410 before the driving of the diffraction grating 21. This allows the trigger signal Strg to be sequentially generated at the timing corresponding to the sampling wavelengths during the driving of the diffraction grating 21. This enables the optical spectrum to be continuously measured by the number of measurement points set in advance.
In JP-A-2000-314661, the optical spectrum analyzer is described.
However, in order to attain a resolution of about 1 pm in the optical spectrum analyzer, an angular resolution of the diffraction grating 21 of about 0.16 seconds (1/8000000 rotations) is required. Therefore, in case where the sampling wavelength range is 1000 nm, diffraction-grating angle data Dsmp of 20 bits or more is required. Moreover, in case where the number of measurement points is set to 50001 points, a memory 410 of 1 Mbits or more is required to store the target diffraction-grating angle data. That is, the number of measurement points is limited by the capacity of the memory 410.
Moreover, the incremental counter 430 and the comparator 420 are made of logic circuitry such as an FPGA (Field Programmable Gate Array). On the other hand, to provide the memory 410 of 1 Mbits or more inside the FPGA, a large-capacity, expensive FPGA needs to be used. The memory 410 is thus made of a memory IC disposed outside the FPGA.
However, in this configuration, the memory 410 (memory IC) is controlled by the FPGA. Thus, as the memory 410, use of an expensive SRAM (Static Random Access Memory), which does not need refresh operation, is needed in place of an inexpensive DRAM (Dynamic Random Access Memory). Accordingly, the above-described configuration brings a cost increase.
Moreover, in the above-described configuration, the target diffraction-grating angle data Dsmp at all the measurement points needs to be written on the memory 410 before the measurement start. Thus, it takes time to start the measurement.
As another configuration, a configuration can also be considered, in which the target diffraction-grating angle data Dsmp is stored in the calculator unit 500, and in place of including the memory 410, a register of the same number of bits as that of the target diffraction-grating angle data Dsmp is included in the FPGA. In this configuration, every time one trigger signal Strg is generated, an interrupt is outputted to the calculator unit 500 to update the value of the register.
However, in this configuration, even when the connection between the control unit 400 and the calculator unit 500 is a Peripheral Component Interconnect (PCI) of 33 MHz, and interrupt processing is performed inside a PCI driver, about 8 μs as is required to recognize the interrupt. Furthermore, for the writing of the data, 1 μs is required.
The data acquisition time interval of the optical spectrum analyzer is at least about 50 μs. Accordingly, in this configuration, in the case that the data acquisition time interval is short as just described, the load on the calculator unit 500 is larger, because the interrupt processing is frequently performed. Thus, there is a possibility that a delay occurs in correction processing or rendering processing.
An object of one aspect of the present application is to realize an optical spectrum analyzer that can reduce costs and time required to start measurement by, for example, reducing hardware such as an SRAM, and can shorten a data acquisition time interval without causing delay in rendering or the like.